The present invention relates to a multi-layer butt welding process for a coated steel produced by forming a diffusion coating with aluminum or chromium on a chromium-molybdenum steel by applying cementation thereto.
Heretofore, as the butt welding process for such a coated steel as above, a process comprising first welding the diffusion-coating side of the coated steel with a 13 Cr ferritic stainless steel and then completing the welding with the same material as the base metal of the coated steel has been popular. However, this conventional welding process is defective in that the corrosion resistance of the weld zone would lower according to the kind of the base metal; for instance, when the concentration of chromium in the base metal is relatively low, despite the welding is effected with a 13 Cr ferritic stainless steel, the concentration of chromium in the weld zone lowers to 10% or thereabouts, so that retention of a satisfactory corrosion resistance of the weld zone cannot be expected. As an alternate process, there has been proposed a multi-layer welding process (See Japanese Pat. No. 36814/1973). This process comprises: first welding the diffusion-coating side of a coated steel with an austenitic stainless steel, next welding it with a welding rod consisting of pure iron, and lastly welding it with the same material as the base metal. However, this process entails various problems such that because of the first layer of the weld zone being formed by employing an austenitic stainless steel, said weld zone is vulnerable to the stress corrosion cracking and/or thermal fatigue, and the employment of pure iron for the second layer thereof renders it substantially impossible to realize the homogeneity in composition of the part inside the weld zone, the weld zone and the base metal, resulting in the fear of bringing about a defect in respect of the strength of the weld zone as a whole.